The invention is generally related to the reduction of vortex induced vibrations or motions in a structure due to fluid flow around the structure and more particularly to the reduction of such vibration or motions through the use of strakes on the structure.
Vortexes are formed in a fluid, when the fluid passes around an object. Under the right conditions, a succession of vortexes can be formed that alternate from side to side of the object. The alternating vortexes produce a pressure variation that alternates from side to side of the object. The alternating pressure variation creates an alternating force on the object in a direction transverse to the flow of the fluid. The period of the alternating force could be resonant with the natural period of the object. Then resonance can damage or destroy the object. Resonance might destroy the object by producing forces strong enough to overload the object, or resonance might produce cyclical loadings that fatigue the material of the object and cause failure over a longer time. Many commercially important objects have been damaged or destroyed by what has come to be known as, “Vortex Induced Vibration”, or VIV.
Almost any object in a stream of fluid might be affected by VIV. Most commonly affected are cylindrical structures, or cylindrical elements of structures. Normally, the fluid is air in the onshore environment, and water in the offshore environment. Some examples of structures that often have VIV related problems are smokestacks, vertical vessels, and elements of truss structures. In the offshore environment in water some examples of structures that often have VIV related problems are risers, TLP tendons, mooring lines, and elements of truss structures. Risers and TLP tendons are pipes that extend from the mudline to the vicinity of the water surface, usually under high tension. They act like giant violin strings plucked by alternating vortexes.
VIV can affect structures of all sizes. One of the largest structures that has a demonstrated response to vortexes is the offshore spar buoy. A spar buoy has a large diameter vertical cylinder moored in the ocean, so that the ocean currents produce vortexes that cause the entire structure to move cyclically, transversely to the current. This motion does not harm the spar itself, but it can overload or fatigue the elements attached to the spar, such as the mooring system, and the risers. This cyclical motion is often called “Vortex Induced Motion”, or VIM. In the case of VIM the whole object moves cyclically as a rigid body, whereas in VIV the object is distorting and vibrating.
There are two general approaches to eliminating or minimizing problems related to VIV or VIM. One approach is to change the period of the structure so that the structural period is no longer resonant with the expected vortex shedding periods. Usually, the structural periods are reduced by making the structure, or its elements, stiffer. Alternately, the structural periods can be increased by adding mass in selected locations to avoid resonance. The second approach is to avoid resonance by affecting the formation of the vortexes.
A method for affecting the formation of vortexes is to install “strakes” on the surface of the structure, or some of its elements. Refer to FIG. 1, which shows three strakes 1 wound in a spiral pattern around a cylinder 2. At any given point along the axis of the cylinder 2, the strake 1 extends radially outward from its contact with the surface of the cylinder 2. If three strakes 1 appear in a cross section through the cylinder 2, in the jargon of the trade, it is said that there are three starts. Sometimes four starts are used, as in FIG. 2. Any number of starts might be used. The “rise” of a strake is the distance it travels along the axis of the cylinder during a full revolution around the cylinder. The “pitch” of a strake is the rise divided by the circumference of the cylinder. The radial projection of the strake from the wall of the cylinder to its tip is most often called the “width” of the strake. The strake width divided by the diameter of the cylinder is an important design variable and is often called the “ratio” and is usually expressed as a percentage.
In a strake design the most important variables include the number of starts, the pitch, and the ratio. In important new designs of strake systems the design is normally confirmed by empirical testing. The ratio tends to vary from 10% to 15% and three starts are most common. The strake is normally made from flat plate that is cut and twisted so that it runs up the spiral and is welded in place. A radial section through a typical strake is shown in FIG. 3. On smaller cylinders the strake is not very wide, so it can be extruded out of plastic and bonded to the cylinder. However, when strakes are applied to large cylinders in water a significant structural strength problem asserts itself. Because the cylinder is large, the strake must be wide. The water exerts large pressures normal to the surface of the strake. The pressure is caused by the action of the waves, the ocean current, and the motion (if the strake is attached to a floating body) of the structure. The strakes must be designed to resist these pressures.
FIG. 4 gives an example of a wide strake that was designed to fit on a large diameter cylinder, moored in the ocean. The simple one plate design of smaller strakes has been abandoned in favor of a stiffened two plate arrangement that allows the strake to resist the applied pressure as a cantilever. The strake shown was about 10 feet wide. FIG. 5 shows an existing design for an even wider strake. This strake is about twenty feet wide. Again, the design develops sufficient strength for the strake to act as a cantilever. Since these strakes were not the typical flat plate strakes, it was thought the strakes might behave differently than predicted by empirical formulae that were developed using single flat plate strake designs. However, testing indicated that these strakes behaved about as predicted. But the newer, stronger designs indicated in FIG. 4 and FIG. 5 did have some disadvantages.
The strake plates 3 forming the surface of the strake in FIG. 4 and FIG. 5 are continuous plates spiraling around the cylinder. These two strake plates 3 support each other and develop the strength of the strake as a cantilever. However, to resist the pressure exerted by the water the two strake plates must be supported locally by stiffener plates 4, which are placed inside the volume enclosed by the strake plates. There are other stiffener plates in the enclosed volume that are not shown for reasons of simplicity. The stiffener plates are placed inside the enclosed volume to reduce the fluid drag that would act on the stiffeners if they were placed on the outside. As can be imagined by studying FIG. 4 and FIG. 5, these strakes are heavy, difficult to build, and expensive. First, men must enter the enclosed volumes to do the necessary fitup and welding of the stiffeners. Second, the fabrication and twisting and fitup of the strake plates 3 is difficult and time consuming.